1. Field of the Invention
The present invention relates to a solid state image sensor with a signal line potential adjustment circuit, and a method of driving the same. In particular, this invention relates to a technique of suppressing a decrease in magnitude of a signal in a unit cell when intense light has been incident on the solid state image sensor.
2. Description of the Related Art
There is known a conventional solid state image sensor having an amplifier in an image sense region. In this solid state image sensor, there occurs such an undesirable phenomenon that the magnitude of a signal from a unit cell has decreased when intense light has struck on the image sensor. With the phenomenon occurring, an image darkens as if no light were incident.
The phenomenon occurs because a signal line potential lowers at the time of a reset operation that is executed prior to the read-out of a signal from the unit cell. At the time of the reset operation, the signal line connected to each unit cell is fixed at a predetermined reference potential. The signal read out of the unit cell is detected on the signal line as a potential difference from the reference potential. However, if intense light has struck on the solid state image sensor, photoelectric conversion takes place in parts other than a photodiode that reads an image. The photoelectric conversion produces excess electrons, resulting in a decrease of the signal line potential from the reference potential. Consequently, the magnitude of the read-out signal from the unit cell decreases as the incident light increases, and the image becomes darker.
A means for solving this problem is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-287131, for instance. In the technique in this publication, a voltage generator for generating a reference voltage is provided. In addition, a comparator is used to monitor whether the signal line potential at the time of reset is within a predetermined range. If the signal line potential has decreased below a predetermined range, a pulse synthesizer generates a signal-pulse. When the signal-pulse has been generated, the reference voltage supplied from the voltage generator is applied to the signal line through a selector. Thus, the signal line potential at the time of reset can be fixed at the reference potential, and even when intense light has been incident, a normal image can be captured.
In the technique of the above-mentioned publication, however, the number of structural elements to be newly added to the solid state image sensor, such as the pulse synthesizer, voltage generator, etc., tends to increase. As a result, the size of the solid state image sensor, as well as the manufacturing cost thereof, tends to increase.